This invention relates to metal shells used to form ends of can type containers. Many can type containers, for example beer cans and soft drink cans, are required to withstand internal pressure, rough handling, and substantial temperature differences, yet maintain a complete hermetic seal to protect the contents of the can. Cans of this type are used in very large volumes, billions of cans per year, and at present the metals most used for this purpose are aluminum and steel.
The typical modern can consists of a unitary deep drawn body, usually with a necked inward throat at the top which terminates in an outwardly extending body curl, and an end for the can which comprises the shell (to which the present invention pertains) provided with self-opening structure such as tear tabs and related score lines in the shell. The shells are manufactured from sheet metal by severing a suitable blank from a strip thereof, forming the blank to define a central panel surrounded by a reinforcing countersink and chuckwall configuration, and a shell curl which is designed to interact with the body curl in seaming apparatus, to attach the end to the can with the requisite hermetic seal. In most instances the underside of the shell or end curl is provided with a sealing compound to assist in the formation of the seal.
The shell is the basic part of the end and is operated upon in converting apparatus which adds the desired score lines, tear tab, and the integral rivet attachment between the shell and the tab, all in known manner. The sealing compound may be applied to the underside of the shell, specifically to the downward facing or bottom portion of the shell curl, either before or after the converting operation, or after, the former being more typical.
One of the major endeavors of designers of can ends is to provide a shell of as thin material as is possible, since this can result in substantial savings of material, and therefore expense. However, the integrity of the shell, and its ability to withstand buckling from internal pressures in particular, impose restrictions upon the use of very thin material in the shell formation. The ability of the thin metal to withstand the drawing and working imposed upon the blank during the formation of the shell generally calls for use of somewhat thicker metal, in order to accommodate thinning in the region where the reinforcing structure is formed in the shell.
In typical prior art operations for the forming of shells, a blank is severed from metal sheet material and it is then formed to a shape comprising a generally flat central panel and a chuckwall extending, in this initial stage, upwardly and outwardly from the central panel, blending into a curved flanged portion. In one prior art method the blank is formed to include a groove around the central panel inward from the chuckwall. This initial blank is then subjected to a rotary curling operation to form a curled edge on the flange, the curled edge being turned somewhat under the flanged portion.
From the curling operation, the partially formed shells are fed through further tooling where they are gripped in the flange portion, while the curled edge is protected in the tooling against deformation. If the groove is already in the blank, then the groove may be reformed. If not, the thus clamped blank is moved against a stationary support applied against the major underside of the central panel.
There is an unsupported region in the shell comprising the edge of the central panel which overlaps and extends beyond the stationary support, out to the region where part of the chuckwall is clamped. This action places the blank in compression, and results in a reshaping of the unsupported band of material between the chuckwall and the central panel, into a shape which defines a reinforcing channel or countersink at the bottom of the chuckwall and into the periphery of the central panel. Thus, the formation of the end shells according to the prior art requires a three stage operation including in some cases a rotary curling step, and the above described formation of a reinforcing channel shape into the shell results from a working of a band of the metal blank between the chuckwall and the central panel which is essentially uncontrolled and thus susceptible to breaks, distortion, or potential thinning of the shell at this critical point in its structure.